Head to the Southeast from the trading post in Concord. You have a good ways to go to reach the marker. There is no fast travel option immediately presented. The best thing to do is either run there or find a horse and ride it there. You will find Ziio there. Talk with her to begin the mission itself.

Optional Objective: Do not kill any guards

This is a tricky optional objective. You must maximize your use of coverage to pull this off and move carefully but quickly. Head to the East quickly over to the road to find a cart passing by. Kenway needs to make it into the cart to get into the fort itself. Just wait for a moment there for it to pass by then slip into the back of it. Now wait until it makes it inside the fort before making any other moves. Once the drive presents their inventory it is only a short wait to get inside. Kenway will make note of something shortly that you will want to deal with for completion.

Optional Objective: Sabotage the 2 cannons.

Once the wagon stops wait a moment then slip out the back and into the stalking area immediately behind it. To the South of where the cart stops you will find an eavesdropping opportunity presented to you.

Slip to the South between the wagon and the tree in the stalking zone and then cross quickly over to the next stalking zone behind the wooden wall South of you. This will start an eavesdropping sequence and the guards are moving to make matters worse.

Keep to the edge of the stalking zone as much as possible and move quickly. Turn back to the North and re-enter the first stalking zone. Turn to the right and quickly duck into the haystack. You are likely to draw a little attention at this point. Move from the haystack into the stalking zone. Wait for the soldiers to move up the stairs then move through the stalking zone and quickly climb the ladder onto the roof of the nearby building. Get to the very edge of the building. You will hear just enough of the conversation to trigger the next main objective at that point. There is a slim chance of glitching the conversation and making it very easy to eavesdrop on if you can get spotted by the guards when crossing from the stalking zone near where the conversation spotted and lose them by moving into the first stalking zone by the cart. Be sure that you are on the opposite side of the tree as them behind the cart in the stalking zone.

Now for the cannons. Return to being nearby the cart by leaping from the rooftop. Head to the Southwest and into the nearby stalking zone. It will lead you most of the way to the first of the cannons without any chance of detection. Follow it to the Southwest and you will find yourself at the end of it with a single guard patrolling the area. Wait for his back to be turn then quickly cross underneath the overhang to the South of you and into the nearby haystack. From there slip into the stalking zone while the guard's back is still turned. Wait for the guard to depart from the cannon then approach and interact with it to sabotage it.

Now head to the East, careful of the eavesdropping to avoid it until you have completed this second part. Head into the stalking zone just to the left of the cannon and once again wait for the guard to pass you by. Head back into the stalking zone you used to cross over here and use it to get back to the starting area. Now turn your attention over to getting back onto the roof and then along the wall toward the second cannon. Hide in the stalking area and watch the guard's patrol in the surrounding area. Wait for the guard above to turn away from the cannon. Run over and interact with it quickly then turn around and hide in the nearby stalking area below where the guard is.

If you had to start at the second cannon, sabotage it then slip over the wall it is out over and start to the right. Hug the fort wall and it will take you directly to the other cannon with minimal chance of detection. Once again, sabotage the cannon after the guard heads away from it and head into the stalking zone on the left. Take it back to the wagon that you started at and slip up the ladder to the area above. From there head into the stalking area and go over to the hay cart near where the guard is patrolling. Whistle to draw him close then knock him out with the action button. This clears the way for you to make it into the command tent and steal the plans. Now head back out into the stalking zone and it is time to escape from the fort.

For this you can either just run out and keep running until you lose them, which takes time or just get to the entrance and exit using the stairs and jumping out without alerting the guards. Either way it is time to return to Ziio. The two make a plan to strike on Braddock's forces as they head for the fort. Things are looking good.